This invention relates to an interfacing circuit for interfacing a telephone system, such as a cordless telephone, with telephone transmission lines. More particularly, this invention relates to such an interfacing circuit which uses optically-coupled transistorized elements rather than hybrid transformers and relay components. Still more particularly, this invention relates to an optically-coupled interfacing circuit of the type described having three optically-coupled elements, each having a light-emitting component and a light-responsive component, for providing DC line loop regulation, incoming audio, outgoing audio, ring detection, and pulse dialing.
In the art of coupling telephone systems to telephone lines, it has been a customary practice to use a hybrid transformer to balance the interfacing of the telephone with the telephone line. However, such a component is bulky and expensive and its associated relay often generates audible noise during the dialing pulse duty cycle. Thus, it is a problem to provide interfacing networks which meet regulatory specifications for voltage isolation, transient suppression, DC current maintenance regardless of loop length, and impedance requirements as are more specifically set forth in the regulations of the Federal Communications Commission, Part 68, while attempting to minimize the circuit size for use in miniaturized components specifically suited for portable use.
Such a problem has become increasingly apparent in developing cordless extension telephones in that the base unit for the cordless telephone requires transmitting and receiving circuits for transmitted connections between the base unit and the portable telephone. Thus, it is desirable to achieve the interfacing function, while meeting federal regulations, in the smallest amount of space consistent with compliance with the regulations to permit incorporation in the base unit of the component pertinent to the cordless telephone operation.
The art of interfacing telephone systems has in the past used opto-couplers. For example, U.S. Pat. No. 4,056,719 utilizes a pair of opto-couplers to achieve electrical isolation between input and output signals without the use of a traditional hybrid transformer. U.S. Pat. No. 3,410,961 also describes the use of an opto-coupler in a line circuit for connecting a subscriber line to a switching network wherein diodes of the optical coupler are connected with the subscriber line while the phototransistors are interconnected with the switching network. Still further, U.S. Pat. No. 3,848,094 discloses the use of an opto-electronic coupler in a telephone circuit in part for detecting off-hook, on-hook, dial, and hook-flash signals. There, the circuit is connected to provide a high impedance AC signal as opposed to DC, wherein the absence and presence of current flow through the direct current path is used to provide dialing signals.
However, such circuits have not incorporated such optical couplers in cordless telephones or in a combination which provides all of the traditional interfacing functions in an optically-coupled network. For example, it is an objective of this invention to provide such an interfacing circuit which provides DC line loop regulation, incoming audio, outgoing audio, line break, off-hook, on-hook, pulse dialing, ring detection, and incoming audio functions in a circuit which utilizes a minimum of components. Such circuit must also provide the appropriate ring equivalence number and a circuit for detecting a ringing signal on the telephone line to establish the telephone line loop. Once the loop is established, it is a problem in the development of such a circuit to provide audio signals from the telephone system to the coupled telephone lines, and to provide audio signals on the telephone lines to the receiver circuits in the telephone system. Moreover, such a network should be arranged to accommodate either dial pulsing or touch tone dialing capabilities for the telephone system thus coupled.
These and other objectives of the invention will become apparent from the written description of the invention which follows taken in connection with the accompanying drawings.